Various chemical, optical or electronic indicators have been proposed for warning users of disposable personal respirators, powered air purifying respirators, haz-mat suits and other protective devices of the presence of undesired materials. For example, an end-of-service-life indicator (“ESLI”) can warn that a filter element in such a device may be approaching saturation or may be ineffective against a particular material. Patents and applications relating to personal protection or respiratory protection (and in some instances to sensors or indicators in general or to ESLIs in particular) include U.S. Pat. No. 1,537,519 (Yablick), U.S. Pat No. 3,966,440 (Roberts), U.S. Pat. No. 4,146,887 (Magnante), U.S. Pat. No. 4,154,586 (Jones et al.), U.S. Pat. No. 4,155,358 (McAllister et al.), U.S. Pat. No. 4,326,514 (Eian), U.S. Pat. No. 4,421,719 (Burleigh), U.S. Pat. No. 4,530,706 (Jones), U.S. Pat. No. 4,597,942 (Meathrel), U.S. Pat. No. 4,684,380 (Leichnitz), U.S. Pat. No. 4,847,594 (Stetter), U.S. Pat. No. 5,297,544 (May et al.), U.S. Pat. No. 5,323,774 (Fehlauer), U.S. Pat. No. 5,376,554 (Vo-Dinh), U.S. Pat. No. 5,512,882 (Stetter et al.), U.S. Pat. No. 5,666,949 (Debe et al. '949), U.S. Pat. No. 5,659,296 (Debe et al. '296), U.S. Pat. No. 6,375,725B1 (Bernard et al.), U.S. Pat. No. 6,497,756B1 (Curado et al.) and U.S Pat. No. 6,701,864 B2 (Watson, Jr. et al.); US. Patent Application Publication Nos. US 2004/0135684 A1 (Steinthal et al.), US 2004/0189982 A1 (Galarneau et al.), US 2004/0223876 A1 (Kirollos et al.) and US 2005/0188749 A1 (Custer et al.); and PCT Published Patent Application No. WO 2004/057314 A2.